3GPP Rel' 14 discusses several study items including SI on V2X communications, SI on HSPA and LTE joint operation, SI on next generation networks (5G), targeted for standardization in 3GPP Rel' 14 and beyond.
3GPP TR 36.885 TSG RAN Study on LTE-based V2X Services (Rel' 14) [1] describes the operation aspects based on which the following possible scenarios for multi-operator supports need to be considered:                Scenario 1: (shared eNB) Operator A deploys an eNB shared with Operator B.        Scenario 2: (non-shared) Both Operator A and B have eNBs in a specific area. Operator A's UE sends a V2X packet, which is further sent to Operator B's eNB for broadcasting.        Scenario 3: (non-shared) Only Operator A has eNB in a specific area. Operator B's UE can only receive MBMS V2X packets from A's eNB.        Scenario 4: (non-shared) Only Operator A has eNB in a specific area. Operator B's UE can transmit, and receive MBMS V2X packets via A's eNB It may not be a valid use case. Pending on RAN1/2 decision.        Scenario 5: (non-shared, V2X using a dedicated PLMN ID) Both Operator A and B have eNB in a specific area. A's eNB broadcasts its own PLMN ID and the V2X PLMN ID. But B's eNB only broadcasts its own PLMN ID. Operator B's UE can transmit, and receive MBMS V2X packets via A's eNB.        
One main challenge may be related to Scenario 5: how B's UE can use both LTE service and V2X service simultaneously?
3GPP RP-160571 [2] proposes a new SI on HSPA and LTE joint operation. It has been identified that even if 3G and 4G are operating in separate carriers, UE with capability limitation of a single Tx chain (1Tx) for 3GPP RAT(s) may have to dynamically switch the transmission in 3G and 4G for simultaneous 3G/4G services. This implies a need for coordination between the serving 3G and 4G RANs to schedule for UE transmissions.
3GPP TR 23.799 TSG SA Study on Architecture for Next Generation System (Rel' 14) [3] describes many key issues including network-slicing, network capability exposure, interworking and migration. For examples, open questions for the network-slicing key issue include: how to enable operators to use the network slicing concept to efficiently support multiple 3rd parties (e.g. enterprises, service providers, content providers, etc.); how to enable a UE to simultaneously obtain services from one or more specific network slice instances. Here, resource sharing across network slices (including spectrum, synchronization or timing, and hardware/software processing resources of both UE and network sides) may be expected; and hence there may be a need for coordination between the involved serving-RANs belonging to different network slices which UE are getting services from simultaneously. There are many challenging vertical use cases which pose severe problems in terms of practical resource sharing and optimization across network slices. For examples, in automotive or factory-automation use cases, regulators or factory owners may question how the network slicing can guarantee the required QoS for safety critical information or in terms of ultra-high reliability and ultra-low latency while running broadband on the same network.
Based on the above examples, it can be generalized that in future mobile networks UE may be provided with multiple radio connections served by multiple RANs of potentially different administrative network domains, technologies, and/or systems (due to different operators, network slices, RATs, APs or operation modes, etc.) for multiple corresponding services simultaneously. RAN level coordination between those serving RANs of the UE is needed to avoid any possible conflicts in terms of scheduled resources for the UE to transmit and/or receive in different serving RANs, especially when resources (including Tx/Rx chain of the UE and spectrum) may be shared between different serving RANs.
If a UE is equipped with multiple, dedicated and exclusive hardware/software resources corresponding to multiple serving RANs operating on exclusive non-interfering carriers then there may be no need for RAN level coordination. For examples, today's UE may already use one of 3GPP RAT (2G/3G/4G) and some non-3GPP RAT (WiFi or Bluetooth) simultaneously.
In current LTE carrier aggregation or RAN level multi-connectivity, RAN-level coordination between P-Cell and S-Cell is considered as intra-RAN functionality and fully under control of the serving RAN (P-Cell) [4]. There is no need for UE to be involved in that RAN level coordination.
It is also known that possible RAN-level coordination between multiple serving RANs may be realized via either UE or network side; and the former option might be seen as the fastest way for RAN-level coordination. One good example for the UE assisted option can be found in coordination between Uu and PC5 transmissions of UE involved in ProSe communications over PC5 in parallel with regular cellular access communications using same UL carrier resources or single Tx chain of UE [4]. This is based on using possible UE assistance information, indicating about autonomous D2D communications UE is involved to the serving RAN, coupled with prioritization of UE transmissions on Uu vs. PC5 controlled by the serving RAN. However, ProSe D2D transmission of individual UE is rather tightly integrated and controlled by single serving RAN. RAN level coordination for ProSe D2D communications of individual UE with possible multiple serving RANs in multi-operator scenarios has not been addressed much. This is more or less the same as the problem or challenge with V2X scenarios addressed in the previous section.
The SI proposed in [2], R3-160837 on concurrent UMTS CS and LTE PS operation [5] describes, at high level, a possible solution based on few key assumptions and proposals/enhancements. These include RAN-aware 3G CS and LTE PS concurrent operation, based on Dual-Rx/Single-Tx UE wherein UL Tx sharing is enabled by the suitable transmission gaps supported by 3G DCH Enhancement and LTE PS C-DRX operation/features. R3-160837 also considers timing-alignment aspects to operate efficiently in such UL-Tx sharing operation. For that R3-160837 suggests that a simple approach, with minimal impacts to RAN (e.g. no need for inter-RAN coordination/synchronization), is to rely on UE detecting/reporting potential misalignments (e.g. over UTRAN, via existing or new time-offset triggers/indications).